Cyanide zinc electroplating

ABSTRACT

Aqueous alkaline cyanide zinc electroplating baths and processes are provided in which polyvinyl alcohol or a vinyl alcohol copolymer degraded with periodic acid or an alkali metal periodate is used as a brightener, particularly to give improved plating characteristics at low applied currents. Enhanced zinc plates are obtained when the degraded polyvinyl alcohol is used with other brightening additives, particularly a pyridinium compound having at least one polar substituent.

United States Patent 1 Adelman 1' Oct. 30, 1973 4] CYANIDE ZINC ELECTROPLATING 3,318,787 5/1967 Rindt et al. 204 55 Y 41 8 [75] Inventor: Robert Leonard Adelman, L996 11/196 Rushmere 204/55 Y X wllmmgton Primary Examiner-F. C. Edmundson [73] Assignee: E. I. du Pont de Nemours and Attorney-Robert W. Black Company, Wilmington, Del. 22 Filed: June 18, 1911 [57] ABSTRACT Aqueous alkaline cyanide zinc electroplating baths [211 Appl' 154654 and processes are provided in which polyvinyl alcohol or a vinyl alcohol copolymer degraded with periodic [52] US. Cl. 204/55 Y, 204/DIG. 2 acid 0! an alkali metal Periodate is used as 8 bright- 51 rm. Cl C23b 5/10 ener, particularly to g improved plating characteris- [58] Field of Search 204/55 Y, DIG. 2 ties a low pp c nt nha ced zinc plate are obtained when the degraded polyvinyl alcohol is used [55 R f ren Ci d with other brightening additives, particularly a pyridinium compound having at l88t one p018! substitu- 2,828,800 3/1960 Wernlund 204/55 Y X em. 3,296,105 1/1967 Rushmere 204/55 R 12 Claims, No Drawings BACKGROUND OF THE INVENTION and more particularly to aqueous alkaline cyanide zinc electroplating baths and processes.

2. Prior Art Polyvinyl alcohol has been a well-known addition agent for cyanide zinc plating, although not outstandingly effective as a brightening agent when used alone. Oxidizing (degrading) the polyvinyl alcohol with hydrogen peroxide to organic peroxides led to brightener improvement as taught by U. S. Pat. No. 2,928,800, issued Mar. 15, 1960, to Christian J. Wernlund. Oxidizing agents other than hydrogen peroxide, such as organic and inorganic peracids and their salts and metallic peroxides, e.g., sodium peroxide or barium peroxide, and other strong oxidizing agents, for example potassium permanganate, are also stated by Wernlund to be effective oxidizing agents for polyvinyl alcohol. However, the prior art has stated that the current density range at which acceptable bright electroplate is produced by such addition agents and other related materials is quite narrow, see U. S. Pat. No.'3,3l8,787, issued May 9, 1967 to Gustav Lindt and Donald H. Becking.

Blends of polyvinyl alcohol or oxidized polyvinyl alcohol with other active brightening ingredients have been disclosed in the prior art such as taught in the aforesaid U. S. Pat. No. 3,318,787 and U. S. Pat. No. 3,41 L996, issued Nov. 19, 1968, to John D. Rushmere.

4 However, it has been observed in barrel plating carried out at a low applied current that the zinc plate is dull and discolored. Even after a conventional bright dip in nitric acid solution, zinc plate appearance is less bright and more colored than obtained at higher applied currents.

The use of previously described'oxidized polyvinyl alcohol also has difficulties in and of itself. Oxidation of polyvinyl alcohol with hydrogen peroxide tends to be difficult to control, the final product tends to degrade further as evidenced by a substantial viscosity drop and appears to have a complex structure and molecular weight distribution. This in turn appears to produce unreproducible results in the plating bath when different batches of oxidized polyvinyl alcohol are used.

SUMMARY OF THE INVENTION According to the present invention there is provided an improved aqueous alkaline cyanide zinc electroplating bath having dissolved therein at a concentration of about 0.001 to 3 grams/liter the reaction product of the substantially complete reaction of a water-soluble vinyl alcohol polymer selected from the group consisting of polyvinyl alcohol and a copolymer of vinyl alcohol with up to 25 percent by weight of a monomer copolymerizable with vinyl acetate, and a compound selected from the group consisting of periodic acid and a salt thereof soluble in water to an extent of at least 0.01 gram per 100 grams of water at 90 to 95C., to give a percent by weight solution viscosity in water of about 9 to 120 centipoises at about C.

There is also provided an improved process for electroplating lustrous zinc from the above bath.

DETAILED DESCRIPTION OF THE INVENTION The polyvinyl acetate or vinyl acetate copolymer starting material used to prepare the degraded polyvinyl alcohol used herein is readily prepared by batchwise or continuous solution, emulsion or suspension polymerization techniques known to those skilled in the art. The book Vinyl and Related Polymers by C. Schildnecht, Wiley & Sons, New York (1952), p. 328-335, describes such techniques. Generally, the starting material is prepared by the homopolymerization of vinyl acetate or by copolymerizing vinyl acetate with up to about 12 percent by weight of a monomer copolymerizable with vinyl acetate, to give a watersoluble polyvinyl alcohol derivative having up to about 25 percent of the comonomer in the vinyl alcohol polymer. Suitable monomers are ethylene, monovinyl esters of saturated aliphatic carboxylic acids or aromatic carboxylic acids of three to 20 carbon atoms, unsaturated mono or dicarboxylic acids such as acrylic acid, methacrylic acid, maleic acid, fumaric acid and crotonic acid, alkyl esters of such acids wherein the alkyl group is from one to eight carbon atoms such as ethyl acrylate, methyl methacrylate, dibutyl maleate, diethyl fumarate and ethyl crotonate and N-substituted amides of such unsaturated acids such as N-propylacrylamide and N-methylolacrylamide. The actual amount of comonomer used will depend on the particular comonomer, i.e., 5 percent ethylene is useful while 8 percent hydroxyethyl acrylate is useful.

To convert the polymers of vinyl acetate to polymers of vinyl alcohol, saponification, acid or basic hydrolysis or acid or basic alcoholysis techniques can be employed, as is well known in the art. Also, vinyl alcohol copolymers prepared by post-reaction of polyvinyl alcohol with various reagents, such as with aldehydes, sulfuric or phosphoric acid, compounds with active chlorine, monoisocyanates and ethylene oxide, are also within the scope of the invention. The vinyl acetate polymers do not have to be completely hydrolyzed. As much as 25 percent residual acetate groups can be present, depending upon the water sensitivity of the comonomer.

The degraded polyvinyl alcohol plating bath additive is prepared by substantially completely reacting periodic acid. (or the hydrate H 10 or salts thereof with a polyvinyl alcohol as described above, contained in an aqueous solution or cold water slurry so that the final solution viscosity at about 25C. is in the range of about 9 to 120 centipoises for a 10 percent solids solution. While sodium periodate is the preferred oxidizing agent, other alkali metal periodates can also be used. Other salts of periodic acid can be used as long as they are soluble in water to an extent of at least 0.01 gram per 100 grams of water, preferably 0.1 gram, at to C. The reaction of polyvinyl alcohol derived from vinyl acetate. homopolymer with periodic acid or periodate is known in the art and is described in Flory and Leutner, J. Poly Sci. 3, 880 (1948) as a method of determining l,2-glycol content in polyvinyl alcohol. Japanese Patent 3048, issued in 1958 to I. Sakurada and G. Takahashi also describes the reaction of polyvinyl alcohol with periodic acid.

Degraded polyvinyl alcohols made by the above reaction have a singular structure comparable to other degraded polyvinyl alcohols made by reaction with hydrogen peroxide, potassium dichromate, potassium per- 14,0 to bring to 1 gallon manganate, potassium bromate in sulfuric acid or ceric salts, in that for the former no pendant carbonyl groups are found along the chain, as evidenced from the infrared spectrum (no absorption at 5.8-5.95u). Also, the molecular weight distribution as determined by gel permeation chromatography is narrow and unimodal, unlike the distribution of other degraded polyvinyl alcohols. The number average molecular weight found most active in brightening is estimated from the GPC to be from about 4,000 to 17,000. This corresponds approximately to 10 percent by weight solution viscosities in water of about 9 to 120 centipoises as measured with an LVF Brookfield viscosimeter, No. l spindle at 60 rpm.

The degraded polyvinyl alcohols can be used in conventional cyanide zinc plating baths atconcentrations within the range of 0.001 to 3 grams per liter. They can be used with 0.01 to grams per liter of a pyridinium compound having at least one polar substituent attached to the ring such as l-benzyl-pyridinium-3- carboxylate in ratios of 100/ l to ill of pyridinium compound/degraded polyvinyl alcohol. For optimum performance at low current densities, concentrations of about 0.1 to 0.5 gram/liter for the pyridinium compound, 0.01 to 0.2 gram/liter for the degraded polyvinyl alcohol and pyridinium compound/degraded polyvinyl alcohol ratios of 20/1 to 2/1 are preferred.

The degraded polyvinyl alcohols used in the present invention have a number of advantages over other degraded polyvinyl alcohols. First of all, they are less sensitive to changes in concentration. Their primary advantage, however, is the excellent bright, light colored coating obtained at low current density; Further, be-

cause of their stability the rate of consumption in the plating bath is lower than other polyvinyl alcohols.

They are effective in still plating as well as barrel plating and chromate conversion coating formulations are effective on the resulting bright zinc deposits after bright dipping. I

In the examples which follow, the following zinc plating bath was used:

NaOl-l 360 grams NaCN 162 grams Zn(CN), 243 grams Na,C0, 360 grams Sodium pclysulfide 15 drops of a 2 lbJgal. solution- EXAMPLES 1 AND 2 To 267 mls. of the zinc plating bath described above, in an angular Hull cell, was added 1.4 mls. of 2 percent aqueous solution of various polyvinyl alcohols. The cell contained a reciprocating paddle covering approximately 25 percent of the steel panel which acted as the cathode, and the paddle moved at 40 cycles/minute approximately 0.1 inch in front of the panel. A pure zinc anode was used. A current of l ampere was passed through the cell for 10 minutes. The panels were then washed in water and partially dipped in 0.5 percent nitric acid solution (a bright dip) to observe any further enhancement of brightness and color. Results are shown in Table l.

cps. =9entipoisas.

, TABLE I Panel character Dull, smoky, or Hazy- Clearpaint bright bright brush" Example range, range, effect, number Brightetler ampsifls. amps/ft. amps/ft Control (a)..- No additive None None 50-1 Control (13)-. PVA MW N 21,000, 1% ace- 40-15 N one 15-0 tats groups present. Degraded with H102 to a viscosity of 11 cps. 3 10% soln.). Control (0).. P A MW 43,000, 1% ace- 40-30 30-25 1 25-0 tats groups present. Degraded with H201 to a. viscosity of 66 cps. (1) PVA, as in (c), degraded 40-30 30-12 12-0 with N8I04 to 38 cps. (2) PVA, as in (c), degraded 40-8 None 8-0 with NaIO; to 77 cps.

1 Bright hut paint brush. PVA=Polyviny1 alcohol; MWN=nurnber average molecular weight;

These results indicate that the hazy-bright plus clearbright region of the panel extends further into the low current density range with the NalO, degraded PVAs than with the other PVAs, when used alone. Further improvement is seen in Examples 3 to 6 below.

EXAMPLES 3 AND 4 To 267 mls. of the zinc plating bath described above, in an angular Hull Cell, was added 0.65 ml. of an aqueous solution containing 10% l-benzyl-pyridinium-Iicarboxylate (BPC) and 2 percent of various polyvinyl alcohols (PVA). The cell contained a reciprocating paddle moving at 40 cycles/minute in front of the steel panel which acted as a cathode. A pure zinc anode was used. A current of 1 amp was passed through the cell for 10 minutes. The panels were then washed and partially dipped in 0.5% nitric acid solution to observe any further enhancement after a bright dip. Results are shown in Table 11:

TABLE 11 Panel Character Clear- Dull, Smoky, I Bright or Paint Example PVA in Brightner Range Brush Effect No. (with BPC), amps/ft. amps/ft. Control (a) No additive 0 50-1 Control (b) PVA LAWN-"6,000, viscosity of 10% soln. 9.0 cps, 12%

acetate groups present 40-18 18-1. Control (c) PVA MW 43,000, viscosity of 10% soln. 900 cps. Degraded with H 0 to viscosity of 11 cps 40-12 12-1 Control (d) PVA as in (c), but degraded with 11 0, to viscosity of 66 cps 40-10 10-1 3 PVA as in (c), degraded with H10 to 33 cps 40-5 5-1 4 PVA as in (c), degraded with NalO to 118 cps 40-6 6-1 The broader brightness range for the brighteners containing polyvinyl alcohol degraded with H10 can be seen from Examples 3 and 4.

EXAMPLES 5 AND 6 A 267 cc. capacity rotating barrel was used with a zinc anode, and loaded with 30 gms. of small wood screws in contact with the steel ring cathode. The applied current was 0.25 Amps for 2 hours. Under these conditions the screws plated using the baths of Control (b) and Control (c) of Table ll, rated only fair to good in brightness and only fair in color. Those using the TABLE III Barrel Runs at Low Applied Current Plating Characteristics (After Bright Dip) Example Color No. PVA Brightness lnhibition Control Degraded PVA's as in Some good Good- Table ll(c) and (d) (some goodexc.

exc.) 5 PVA as in Table ll(c) (degraded with NalO; All good- Exc. to 36 cps) exc. 6 PVA ex 96/4 VAc/MMA" copolymer, then degraded All good- Exc.

with NalO to 18 cps exc. VAc/MMA vinyl acetate/methyl methacrylate copolymer These results show the improved reproducibility of 7 Examples 5 and 6 over the Control in that certain batches of the Control rated only good in brightness, while others rated good-excellent.

What is claimed is: v 1. In an aqueous alkaline cyanide zinc electroplating bath the improvement comprising having dissolved therein at a concentration of about 0.001 to 3 grams/- liter the reaction product of the substantially complete reaction of a water-soluble vinyl alcohol polymer selected from the group consisting of polyvinyl alcohol and a copolymer of vinyl alcohol with up to 25 percent by weight of a monomer copolymerizable with vinyl acetate selected from the group consisting of ethylene, monovinyl esters of saturated alliphatic carboxylic acids or aromatic carboxylic acids, unsaturated mono or dicarboxylic acids, alkyl esters of unsaturated mono or dicarboxylic acids wherein the alkyl group has from 1 to eight carbon atoms and N-substituted amides of unsaturated mono or dicarboxylic acids and a compound selected from the group consisting of periodic acid and a salt thereof soluble in water to an extent of at least 0.01 gram per 100 grams of water at 90 to 95C., to give a 10 percent by weight solution viscosity in water of about 9 to 120 centipoises at about 25C.

2. The electroplating bath of claim 1 wherein the compound is a salt of periodic acid soluble in water to the extent of 0.1 gram per 100 grams of water at to C.

3. The electroplating bath of claim 2 wherein the salt of periodic acid is an alkali metal salt of periodic acid.

4. The electroplating bath of claim 3 wherein the alkali metal salt of periodic acid is sodium periodate.

5. The electroplating bath of claim 1 wehrein the concentration of the reaction product is about 0.01 to 1 gram/liter.

6. The electroplating bath of claim 5 wherein there is also dissolved in the bath 0.01 to 5 grams/liter of a pyridinium compound having at least one polar substituent.

7. The electroplating bath of claim 6 wherein the pyridinium compound is carboxylate.

8. The electroplating bath of claim 1 wherein the vinyl alcohol polymer is a copolymer of about 96 percent by weight vinyl alcohol and about 4 percent by weight methyl methacrylate.

9. In a process for electroplating lustrous zinc from an aqueous alkaline cyanide zinc electroplating bath, the improvement comprising adding to said bath at a concentration of about 0.001 to 3 grams/liter the reaction product of the substantially complete reaction of a vinyl alcohol polymer selected from the group consisting of polyvinyl alcohol and a copolymer of vinyl alcohol with up to 14 percent by weight of a monomer copolymerizable with vinyl acetate selected from the group consisting of ethylene, monovinyl esters of saturated alliphatic carboxylic acids or aromatic carboxylic acids, unsaturated mono or dicarboxylic acids, alkyl esters of unsaturated mono or dicarboxylic acids wherein the alkyl group has from one to eight carbon atoms and N-substituted amides of unsaturated mono or dicarboxylic acids and a compound selected from the group consisting of periodic acid and a salt thereof soluble in water to an extent of at least 0.01 gram per lOO grams of water at 90 to 95C., to give a 10 percent by weight solution viscosity in water of about 9 to centipoises at about 25C.

10. The process of claim 9 wherein the compound is an alkali metal salt of periodic acid.

11. The process of claim 10 wherein there is also added to the bath 0.01 to 5 grams/liter of a pyridinium compound having at least one polar substituent.

12. The process of claim 11 wherein the pyridinium compound is l-benzyl-pyridinium-3-carboxylate.

l-benzyl-pyridinium-3- 

2. The electroplating bath of claim 1 wherein the compound is a salt of periodic acid soluble in water to the extent of 0.1 gram per 100 grams of water at 90* to 95*C.
 3. The electroplating bath of claim 2 wherein the salt of periodic acid is an alkali metal salt of periodic acid.
 4. The electroplating bath of claim 3 wherein the alkali metal salt of periodic acid is sodium periodate.
 5. The electroplating bath of claim 1 wherein the concentration of the reaction product is about 0.01 to 1 gram/liter.
 6. The electroplating bath of claim 5 wherein there is also dissolved in the bath 0.01 to 5 grams/liter of a pyridinium compound having at least one polar substituent.
 7. The electroplating bath of claim 6 wherein the pyridinium compound is 1-benzyl-pyridinium-3-carboxylate.
 8. The electroplating bath of claim 1 wherein the vinyl alcohol polymer is a copolymer of about 96 percent by weight vinyl alcohol and about 4 percent by weight methyl methacrylate.
 9. In a process for electroplating lustrous zinc from an aqueous alkaline cyanide zinc electroplating bath, the improvement comprising adding to said bath at a concentration of about 0.001 to 3 grams/liter the reaction product of the substantially complete reaction of a vinyl alcohol polymer selected from the group consisting of polyvinyl alcohol and a copolymer of vinyl alcohol with up to 14 percent by weight of a monomer copolymerizable with vinyl acetate selected from the group consisting of ethylene, monovinyl esters of saturated alliphatic carboxylic acids or aromatic carboxylic acids, unsaturated mono or dicarboxylic acids, alkyl esters of unsaturated mono or dicarboxylic acids wherein the alkyl group has from one to eight carbon atoms and N-substituted amides of unsaturated mono or dicarboxylic acids and a compound selected from the group consisting of periodic acid and a salt thereof soluble in water to an extent of at least 0.01 gram per 100 grams of water at 90* to 95*C., to give a 10 percent by weight solution viscosity in water of about 9 to 120 centipoises at about 25*C.
 10. The process of claim 9 wherein the compound is an alkali metal salt of periodic acid.
 11. The process of claim 10 wherein there is also added to the bath 0.01 to 5 grams/liter of a pyridinium compound having at least one polar substituent.
 12. The process of claim 11 wherein the pyridinium compound is 1-benzyl-pyridinium-3-carboxylate. 